Noise filters or choke coils used for high frequency transformers of this kind are required to have a substantially constancy of magnetic permeability, that is, a magnetic permeability not greatly depending on the intensity of magnetic field H but remains substantially constant. For satisfying the constancy of magnetic permeability, in a so-called amorphous core made of an amorphous alloy, thin film of a ferrous amorphous alloy (hereinafter referred to as an amorphous ribbon or a magnetic ribbon) Is wound by a required number of turns, applied with a heat treatment and impregnated with an adhesive such as an epoxy resin and, after hardening, a gap for disconnecting a part of a magnetic flux path is disposed to attain the constancy of magnetic permeability.
Since it is expected that the choke coils of this kind will be used in near future in a high frequency region of several hundreds kHz or higher, it is necessary in such high frequency region to minimize the heat generated from the core, that is, the core loss (iron loss) as low as possible.
In view of the above, the magnetic core formed with the gap as described above involves a problem that the core loss is increased remarkably due to insulation failure or the like at the cutting surface, in addition to compressive stresses upon impregnation and hardening of tile epoxy resin and working strain upon cutting.
In view of tile foregoings, various techniques have been proposed for attaining the constancy of magnetic permeability without forming the gap.
In the earliest study made by A. Datta, et al, it was described in "Proc. 4th Int. Conf. on Rapidly Metals" (pp 1007-1010) published in 1981, that .alpha.-Fe fine crystallites are deposited near the surface of an amorphous ribbon after the heat treatment, which provide the constancy of magnetic permeability.
Then, it was proposed in Japanese Patent Laid-Open No. Sho. 63-24016 to apply a heat treatment at a low temperature lower than the temperature for the crystallization for more than 10 hours and stably suppress the crystallization at the surface, to attain the constancy of magnetic permeability.
However, in the above-mentioned prior art, since a core having an aimed constancy of magnetic permeability is obtained by precipitating fine crystallites at the surface of the amorphous ribbon, even a slight temperature change in the heat treatment results in the fluctuation of the magnetic permeability and involves a problem that products of stable quality can not be supplied by a great amount.
On the other hand, for the crystallization at the surface of the ferrous amorphous ribbon, N. Motira et al have reported in J. Japan, Inst. Metals, Vol. 52, No. 4 (1988), pp 420-427, that they found phenomena for the occurrence of crystallization near the surface layer of the amorphous ribbon (Fe-B-Si series) and, at the same time, deteriorating of core loss if water is contained in the heat treatment atmosphere. According to the report, when a thin film amorphous alloy Fe.sub.78.5 B.sub.13 S.sub.8.5 is annealed at 673K, the core loss is improved by the annealing among At, N.sub.2, dry H.sub.2 and N.sub.2 +O.sub.2 and there is substantially no difference between the values of the core loss. However, it is described that the core loss is deteriorated by annealing the amorphous ribbon in a wet H.sub.2 atmosphere with a dew point of 323 K (50.degree. C.). However, the literature mentions nothing about a heat treatment method for attaining a constancy of magnetic permeability.